Electronic Stethoscope 2
Circuit Description:
. U1a operates as a low-noise microphone preamp. Its
gain is only about 3.9 because the high output impedance
of the drain of the FET inside the electret microphone
causes U1a's effective input resistor to be about 12.2K.
C2 has a fairly high value in order to pass very low
frequency (about 20 to 30Hz) heartbeat sounds.
. U1b operates as a low-noise Sallen and Key,
Butterworth low-pass-filter with a cutoff frequency of
about 103Hz. R7 and R8 provide a gain of about 1.6 and
allow the use of equal values for C3 and C4 but still
producing a sharp Butterworth response. The rolloff rate
is 12dB/octave. C3 and C4 can be reduced to 4.7nF to
increase the cutoff frequency to 1KHz to hear
respiratory or mechanical (automobile engine) sounds.
. The U4 circuit is optional and has a gain of 71 to
drive the bi-colour LED.
. U5 is a 1/4W power amplifier IC with built-in biasing
and inputs that are referred to ground. It has a gain of
20. It can drive any type of headphones including low
impedance (8 ohms) ones.
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PRIVATE/Part
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Total Qty. | Description |
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R1 |
1 |
10K 1/4W Resistor |
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R2 |
1 |
2.2K 1/4W Resistor |
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R3, R9 |
0 |
Not used |
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R4 |
1 |
47K 1/4W Resistor |
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R5, R6, R7 |
3 |
33K 1/4W Resistor |
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R8 |
1 |
56K 1/4W Resistor |
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R10 |
1 |
4.7K 1/4W Resistor |
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R11 |
1 |
2.2K to 10K audio-taper (logarithmic) volume control |
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R12 |
1 |
330K 1/4W Resistor |
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R13, R15, R16 |
3 |
1K 1/4W Resistor |
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R14 |
1 |
3.9 Ohm 1/4W Resistor |
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C1, C8 |
2 |
470uF/16V Electrolytic Capacitor |
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C2 |
1 |
4.7uF/16V Electrolytic Capacitor |
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C3, C4 |
3 |
0.047uF/50V Metalized plastic-film Capacitor |
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C5 |
1 |
0.1uF/50V Ceramic disc Capacitor |
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C6, C7 |
2 |
1000uF/16V Electrolytic Capacitor |
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U1 U2, U3 U4 U5 |
1 0 1 1 |
TL072 Low-noise, dual opamp Not used 741 opamp LM386 1/4W power amp
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MIC |
1 |
Two-wire Electret Microphone |
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J1 |
1 |
1/8" Stereo Headphones Jack |
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LED |
1 |
Red/green 2-wire LED |
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Batt1, Batt2 |
2 |
9V Alkaline Battery |
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SW |
1 |
2-pole, single throw Power Switch |
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Misc. |
1 |
Stethoscope head or jar lid, Rubber Sleeve for microphone. |
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Assembly:
1) Assemble the circuit using Veroboard (stripboard) or
a PCB.
2) Use a shielded cable for the microphone as shown on
the schematic.
3) Fasten the microphone to the stethoscope head with a
rubber isolating sleeve or use a short piece of rubber
tubing on its nipple. A thick jar lid can be used as a
stethoscope head. The microphone must be spaced away
from the skin but the stethoscope head must be pressed
to the skin, sealing the microphone from background
noises and avoiding acoustical feedback with your
headphones.
4) The microphone/stethoscope head must not be moved
while listening to heartbeats to avoid friction noises.
5) Protect your hearing. Keep the microphone away from
your headphones to avoid acoustical feedback.